• SLC -> Single-Level Cell, i.e. 1 bit per cell
• MLC -> Multi-Level Cell, i.e. 2 bits per cell
• TLC -> Triple-Level Cell, i.e. 3 bits per cell
• QLC -> Quad-Level Cell, i.e. 4 bits per cell

The more bits per cell you store, the more dense and therefore cheaper your flash chips can be for a give capacity. The downside is that it is slower and less reliable since you have to be able to write and read exponentially more voltage states per cell, e.g. 2 states for SLC, 4 states for MLC, 8 states for TLC, etc.

the timer has no idea if it was triggered during last boot. It only has the context of “this” boot, so it will do it right after a reboot and set a timer to start the service again after a week of uptime.

This is not correct. Persistent=true saves the last time the timer was run on disk. From the systemd.timer man page:

Takes a boolean argument. If true, the time when the service unit was last triggered is stored on disk. When the timer is activated, the service unit is triggered immediately if it would have been triggered at least once during the time when the timer was inactive.

OP needs to remove Requires=backup.service from the [Unit] section so it stops running it when it start the timer on boot.

You have the timer requiring backup.service, so it will run that service every time the timer starts on boot. Remove Requires=backup.service, and that will fix the issue.

Well, for one, it’s network attached storage. If it’s not present in the network for one reason or another, guess what, your OS doesn’t boot… or it errors during boot, depending on how the kernel was compiled and what switches your bootloader sends to the kernel during boot.

Just use nofail in the fstab.

Second, this is an easy way for malware to spread, especially if it’s set to run after user logon.

If your fileshare is accessible to you, it is also accessible to malware running as your user. Mounting the share via a filemanager doesn’t change this.

USB 2 is 480 Mb/s, not 480 MB/s. 480 Mb/s is 60 MB/s, so the 500 MB/s from PCIe 2.0 x1 is quite a bit faster and is about the limit of what a SATA 3 interface could do. Also, sequential throughput isn’t nearly as important as most people think. Random IO, which NVMe drives excel at, will make a far more noticeable impact on real world performance.